Skin-type membranes Skinning, phase Inversion

Symmetric membranes and asymmetric membranes are two basic types of membrane based on their structure. Symmetric membranes include non-porous (dense) symmetric membranes and porous symmetric membranes, while asymmetric membranes include integrally skinned asymmetric membranes, coated asymmetric membranes, and composite membranes. A number of different methods are used to prepare these membranes. The most important techniques are sintering, stretching, track-etching, template leaching, phase inversion, and coating (13,33). 

Broadhead and Tresco studied the effects of fabrication conditions on the structures and performances of membranes formed from poly(acrylonitrile-vinylchloride) (PAN-PVC) by using the phase inversion process [85]. They reported the relationship of the fine-surface structure of PAN-PVC membranes to the membrane performance and membrane fabrication method. The fine-surface structure of nodular elements and the size of these elements could be altered by changing the precipitation conditions. Membranes were prepared at 22 on 55 mm diameter polished silicon wafers by spinning at 1500 rpm for 20 s with a spin coater [86]. The film was immediately precipitated in one of the four different precipitation media. The first three media consisted of deionized water at 4,22, and 54 °C. These membranes were referred to as Type 1 , Type 2 , and Type 3 , respectively. The fourth medium was a 50/50 mixture of deionized water and N,iV-dimethylformamide (DMF) at 54 °C and coded as Type 4 . Figure 4.53 shows the histograms of the nodule size distributions observed at the skinned surface of the membranes made under four different precipitation conditions. The sizes of these nodular elements became smaller and more uniform with milder precipitation conditions, which supports the theory that nodules are formed through spinodal decomposition under these conditions. In addition, the size of these nodules could be related to water permeability. Hence, water transport occurred through the interstitial spaces where the pores could be situated. 

Anisotropic (asymmetric) Defines a particular type of ultrastructure of microporous membranes. The surface of the membrane where separation occurs is more dense than the rest of the membrane body. The pore diameter increases in a direction perpendicular to the membrane surface, with the pore opening near the separation surface being smaller than the pore opening on the bottom of the membrane. This skin layer is typically present in polymeric membranes made by the phase-inversion process. Some asymmetry is also present in many inorganic membranes. 

Asymmetric membranes, which are more important commercially, come in a wide variety of types. I will discuss three types here. The first type of asymmetric membrane is that made by phase inversion. This process, originally developed by Leob and Sourirajan, involves spreading a polymer solution on a moving web. The polymer solution typically contains 20% polymer in a volatile solvent like acetone. Some of the solvent evaporates to form a thin skin of polymer. The moving web then dips into a nonsolvent like water... 

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